First you plug a $70 device, the Automatic Link, into your car's dashboard computer. (Just about every car built after 1996 has the plug, just few of us know it's there.) Then you download a free app and sign in. Then you drive.

The Automatic Link monitors your driving patterns, including your speed, acceleration and braking. It lets you know the value of the gas you're burning with your driving habits, your estimated miles per gallon based on the model of your car, and tells you how you could save. Based on your driving style, it gives you a score — up to 100 — to let you know how you're doing.

But that's just the beginning. Because the Automatic Link communicates directly with your car's computer, it can perform feats that seem magical to the average driver — like telling you exactly why the "check engine" light is on, giving you just enough information to decide whether it's something you can fix yourself.

The app even lets you turn the "check engine" light off from a screen on your phone. (It will come back on if the problem is not fixed.) In the event you get into an accident, the device can detect the impact and automatically call 911.

Why is this so significant? Because the average American spends about 15 hours a week in the car, and until now the car's computer has been a black box. Useless to the consumer. Most of us don't even know it's there.

But beyond that, there's a bigger reason:

We are entering a new era of computing, where the biggest growth won't belong to the smartphone. Instead, it will belong to devices and services that rely on the smartphone as a digital hub.

We've been here before, more than a decade ago, when Steve Jobs declared the era of the PC as digital hub. He decided to position the Mac as the best platform for doing things like organizing and editing digital photos, music and video.

Back then, in the early 2000's, PC growth was moderating, broadband was taking off, and the tech ecosystem was casting about for the next big thing. Digital cameras and camcorders boomed and webcams appeared. Eventually, the biggest Internet accessory of all arrived: the iPod.

It's easy, in retrospect, to imagine that the iPod was an obvious development. Before it arrived (and for quite a while after), we had something like the Hobbesian State of Nature in devices that connected to PCs.

There were MiniDisc players, players with big hard disks, little flash players. What they all lacked was a comprehensive approach to music purchase, management and portability in the digital age. iTunes, with its interface and Music Store, combined with the iPod, solved that.

I've got the strong sense that we're entering a new digital hub era. This time it's not built on PCs and wired broadband connections, but on smartphones and wireless. Sometime over the next couple of years, someone will come up with the killer app for this era. In my conversations with Walmart's head of general merchandising early this year, he thought home security and automation would be a big driver.

At Automatic, a little San Francisco-based start-up, CEO Thejo Kote and his team have tapped into what I'm sure will be another major factor in the post-smartphone era: the car. And the elegance of their Automatic Link device and service suggest to me that they've at least got a valuable head start toward making their clever little product into something very important.

You can pre-order the Automatic Link starting today at Automatic.com, for $70. The smartphone app is free. Kote, Automatic's CEO, tells me the service will be available on Apple's iPhone 4S and 5 users in May, and to Android users later in the year.